The Potters defence has since been hit by injury.
Coates told The Sentinel: "Yes, I can understand why supporters would be asking why did you let him go, but equally I can understand the management team's reasons for doing so.
"They wanted to give him games after his long-term knee injury and it was felt reserve games weren't suitable.
"We had also brought in Philipp Wollscheid to add to the centre-halves we had here.
"The timing was then very unfortunate with the injuries that followed the decision to let Robert go.
"Ryan has got a wonderful appearance record, for instance, but then came that bad run of injuries which included him, Marc Wilson and now Marc Muniesa."
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